Variable voltage transformer



INVENTOR. 4/7/52 A? flw /s Kg h aria/ME) 4 Sheets-Sheet 1 A. R. DAVIS VARIABLE VOLTAGE TRANSFORMER March 24,1953

Flled Oct 18, 1949 March 24, 1953 A. R. DAVIS 2,632,857

VARIABLE VOLTAGE TRANSFORMER Filed Oct. 18, 1949 4 Sheets-Sheet 2 March 24, 1953 A. R. DAVIS 2,532,357

VARIABLE VOLTAGE TRANSFORMER Filed Oct. 18, 1949 4 Sheets-Sheet 5 IN V EN TOR. .Amzz R J m;

Ale Afro/WW3! h 4 sheets-sheet 4 IN VEN TOR.

Wm i Z Z n ,z'rck- A. R. DAVIS VARIABLE VOLTAGE TRANSFORMER /U V V U T U U U V Hill/Ill! l uuvvv g I Mk3 WW March 24, 1953 Filed Oct. 18, 1949 Patented Mar. 24, 1953 UNITED STATES PATENT OFFICE 15 Claims.

This improvement relates to transformers and associated parts for changing the voltage applied to one or more load circuits over a Wide range of a constant voltage alternating current source from the full applied voltage to zero voltage or a close approach thereto. It is particularly applicable to the control of stage lighting circuits in theatres and the like, although applicable to various other uses where a variable voltage control is desired.

This invention is an improvement over the disclosures of my prior Patent No. 2,361,384, granted October 31, 1944; and is a continuation in part of my co-pending applications Serial Nos. 80,479, now abandoned, and 80,480, filed March 9, 1949.

One object of the present invention is to reduce the heating of the transformer winding particularly that which occurs at the time the movable contact short-circuits one or more turns of the winding when bridging two adjacent contacts. Another object is to reduce the sparking at the brushes and thereby insure longer life of the parts. A further object is to provide a structure for enabling several lighting or distribution circuits to be supplied from a single transformer unit, each of which circuits may be variably controlled independently of each of the other circuits or all of which may be simultaneously and similarly controlled as desired.

An important object is to increase the refinement of control throughout the full voltage range by introducing intermediate steps of control between the change in passing from one step of the transformer winding to the next step of the transformer winding. A further object is to relate the brushes supplying several circuits to the turns of the transformer winding in such a manner as to avoid the possibility of all brushes of the several circuits contacting the same turn of the winding at the same time. This avoids the possible concentration of the maximum heating on one turn and thereby increases the capacity of the unit. Another object is to relate the associated parts to the transformer in a manner to obtain a compact and unitary structure and to dissipate the main portion of the heat at a distance from the winding and thereby further increase its capacity. Another object is to provide an improved brush assembly particularly adapted for the type of control utilized herein and to provide simplified connections therefrom to the associated parts. A further object is to provide extra transformer turns connected in the individual load circuits for increasing the circuit voltage to compensate for drop of volts therein which thus increases the capacity of the main winding by avoiding extra, turns thereon and also permits the full extent of the main winding to be utilized for variation of the output voltage. Another object is to provide ,a. unitary structure particularly adapted for use where a higher voltage is available than the usual volt circuit, such as 220 volts, and where the load circuit is to receive a lower voltage, such as a maximum of 110 volts, by adding an: other winding on the transformer core suitably connected to the main winding and to the load. These and other objects and advantages will be understood from the following description and accompanying drawings which illustrate preferred embodiment of the invention.

Fig. 1 is a side view of a structural embodiment of one form of this improvement partly in section and with the enclosing casing removed excepting the top cover plate; Fig. 2 is an end view of the interior, partly in section; Fig. 3 is a top plan view; Fig. 4 is a vertical section of the upper portion of the unit showing the several brush assemblies and their engagement with the contact surfaces of the winding and the form of the winding in the contact region; Fig. .5 is a vertical section of one of the brush assemblies showin its manner of engagement with the contact surfaces of the winding; Figs. 6 and 7 are enlarged sectional views of the engagement of the contact parts for explanatory purposes; Fig. 8 is a diagram of the connections of the parts shown in the structure of the prior figures; Fig. 9 is a diagram of connections showing additional features of this improvement; Fig. 10 is a cross section of a core showing another form of winding; and Fig. 11 is a sectional view of a portion of a core and winding showing an added contact bar to a turn of the winding for increasing the contact area available for engagement by the brushes.

Referring to Fig. 1, the unit comprises a laminated iron core of rectangular form having an upper leg i and a lower leg la. joined at their ends by two short sections On the pper leg is a main winding 2 extending over its full length. The upper leg has two paths of contact surfaces formed on portions of the main winding and is of a form shown in Logan Patent No. 2,189,507, granted February 6, 1940. That is, as shown in Fig. 4 of this application, the top portion of each turn at one side has an exposed contact surface whereas at the other side the other portion of the turn is positioned under the exposed contact portion of the next turn. Thus in Fig. 4, the turn 20!, at the left half of the top portion has an exposed contact surface whereas the righthand half is located under the exposed portion of the next turn. This next turn 21) has its exposed portion at the right and is under the turn 2a at the left and so on. This results in two contact paths along the length of the winding spaced somewhat from each other. Each of these paths is engaged by one or more brush assemblies each of which supplies a load circuit independently of each of the others. It may be noted here that all of the brush assemblies cannot be positioned on the contact surface of any one turn of the winding at one time to concentrate the heating thereon, because the brush assemblies engaging the other contact path cannot be concentrated on the same turn engaged by the brush assemblies of the other path as they necessarily must engage the next adjoining turn in the nearest approach thereto. The concentration of the maximum possible heating on any one turn is thereby avoided with a resulting increase in capacity of the unit. Each brush assembly, as shown in Fig. comprises two sheet metal strips 3 and 3a spaced apart from each other by a strip 3 of insulating material. Fig. 5 shows a side view of the spaced apart brushes 3 and 3a, with their lower ends in engagement with a portion of one of the contact paths of the winding. The spacing of the two brushes is such that contact is always made with some portion of the winding as it is moved along the path. In Fig. '5 a top portion of one of the contact paths is indicated with a cross section of the turns .2 shown as rectangular and having insulating spacing strips 5 between them. Fig. 5 shows the two brushes 3 and 3a in engagement with the contact surface of one turn of the winding, shown enlarged in Fig. 6. As the brush assembly is moved along the path, the brush 3 will be in engagement with a contact surface of one turn of the winding while the brush Set will be in engagement with an insulating strip 5. Further movement will cause the two brushes to bridge an insulating strip and simultaneously engage the contact surfaces of two turns of the winding, as shown in Fig. '7. Further movement causes one of the brushes toengage an insulating strip while the other brush continues in its engagement with its turn of the winding and so on. The mode of operation in this series of changes of brush assembly positions will be explained later. It will also be understood that the main winding 2 with its two paths of contact surfaces may be formed in various ways, such as by an insulated wire and then grinding off portions of the insulation to form the exposed contact surfaces, or the winding may be formed of bare wire and positioned in a formed plate of insulating material with insulating strips between the turns, as described in said Logan patent. Likewise the cross section of the winding may be round, square, rectangular or of other form for suitably forming the contact surfaces and, of course, the two paths will be ground to form a smooth surface for engagement by the brushes.

As shown in Fig. 5, each brush assembly comprises a block 6 of insulating material which forms the main support for the parts. Each brush has a portion bent at right-angles at its upper end which is engaged by a spring 1 positioned above the brush in an opening in the block -6 for the purpose of maintaining good contact engagement with the exposed portions of the winding. The brushes may be made of strips of copper, or preferably silver and each brush is engaged at one side by a contact block 8 having a contact portion Ba, preferably of silver soldered to the block and directly engaging its brush. This block is seated in a horizontal circular opening in the support 6 and is pressed against the brush by a spring 9 which is retained in position by a screw plug it at the outer end of the opening. This arrangement insures that the two brushes of each brush assembly are forced toward each other against the intervening insulating strip 4 and thereby results in a proper spacing of the two brushes from each other at all times. At each end portion of the insulating support is shown an insulated flexible cable H which enters an opening in the upper part of the support. At the inner end of the cable the insulation is peeled 01f and its flexible wire Ha is bent on itself and passed outwardly from the opening and then turns downwardly and passes through the block it and is electrically connected by soldering or otherwise to the block 8. In this manner good electrical con nection is made rom each of the brushes to its respective cable. Each cable is held in place at the top portion of the support it by a screw plug i2 extending downwardly from the top of the insulating block to firmly engage the inserted end of the cable ii and hold it in place. In the top central portion of the block 6 and extending along a portion of its length is formed a slot for receiving a metal plate I3 which extends upwardly through a cover 84 and is secured to the insulating block 6 by cross pins l3a. A handle E5 of insulating material above the cover M is slotted in its lower portion to receive the plate l3 and is secured thereto by cross pins Ilia. The cover M is provided with slots [4a parallel with each other, as shown in Fig. 3 for permitting each brush assembly to be moved individually along its contact path of the winding. As well shown in Figs. 2, 3 and i, there are a total of six brush assemblies in this particular disclosure, three of which are adapted to have contact engagement with one of the contact paths of the winding and three of which are adapted to engage the other contact path of the winding.

The unit is enclosed by a suitable frame or box not shown and the cover I4 is supported thereon and secured thereto. A pair of screw bolts [6 pass downwardly through the cover and engage cross pieces ll respectively at opposite ends of the leg I of the core, these cross pieces being fixed to the core. The cover and parts carried thereby may be readily removed from the unit by merely removing the screws it. As shown in Figs. 2 and 4 there is secured to the under side of the cover it in slots therein a series of plates I8 of insulating material. These plates extend the full length of the core as shown in Fig. l and extend downwardly between the brush assemblies to within a short distance of the contact paths of the winding. They serve to properly guide the brush assemblies in their movement along the contact paths and also insulate each brush assembly from each other. As shown in Fig. 1, the flexible cables H extending from opposite sides of each brush assembly pass through the spaces between the plates 18 and then are looped downwardly and upwardly around supporting rods i9 at opposite ends of the core, from which they pass downwardly and continue in the manner later described. These supporting rods l-9 are held in fixed position at their opposite ends by metal strips I90, which are in turn secured to the core. These rods carry a series of spools l9b which are spaced from each other by intervening sheets 29 of insulating material which are also carried and supported by the rods I9. Immediately below the spools lQb cables H are drawn together and held in place by wire clips I lb which serve to hold this portion of the cables in fixed position on their supporting spools. A triangular guide block 2| is fixed to the cover H! at each end of the unit and has an inclined surface over the looped portions of the cables II. It results that when the brush assemblies are moved to change the voltage applied to the load circuit, the looped portions of the cables H are properly guided by the blocks 2! and by the insulating plates 20 between each of the looped portions.

At each end of the core are plates or strips 22 which serve to clamp the laminae of the core together by through bolts and these clamping strips are extended downwardly as particularly shown in Figs. 1 and 2 to form a support for the unit at the base by the lower portion of the U-shaped strips 22. Below the core are a series of impedance devices indicated as wire wound resistors 23, six of them being shown in Fig. 1 to correspond respectively with the six brush assemblies. Fig. 1 shows the connections between one of these resistors and the two brushes of one of the brush assemblies. The flexible cable I i from the left-hand brush of Fig. 1 passes around the fixed support 19 and then continues downwardly and around the lower leg la of the core with one turn 28 thereof, then passes to a terminal of one of the resistors, then from the other terminal to form another turn 26a around the leg la, then around the fixed support 19 at the right of Fig. l and then by a looped portion to the other brush of its brush assembly. Each resistor is shown provided with a connection 23a which extends from the junction where the lead II is connected to the resistor after making the first turn from the leg la. The six resistors are supported at their ends between metal strips 24 which in turn are fixed at their ends to the strips 22. Although the connections between only one brush assembly and its resistor is indicated in Fig. 1, it will be understood that the same form of connection is made from each of the brush assemblies to each of the resistors respectively. It will be understood that each of the brush assemblies may be moved along its contact path on the main winding throughout its full length manually by means of the handles [5 independently of each other to change the voltage applied to the load circuit and cocupy any selected position respectively, or they may be moved simultaneously if desired. As each brush assembly is moved it is guided by the intervening insulating plates l8. Also as each brush assembly is moved, it will draw the cable or lead II from one side along with it and force the cable at the other side to increase the size of its loop, the loops being independently guided and maintained in proper relative positions by the plates 29 and the guide blocks 2|.

Although a total of six brush assemblies is shown in the drawings, it will be understood that a greater or lesser number thereof may be used as required and depending upon the widths of the contact surfaces of the two contact paths of the windings provided therefor. Also, instead of using the particular form of winding de- 6 scribed with theprovisionof two contact paths, only one contact path may be provided with a more simple and usual form of structure if desired.

The connections of the parts for the particular disclosure already described is shown diagrammatically in Fig. 8. Here an alternating source 25 is indicated from the upper terminal of which a connection extends to the right-hand side of the main winding 2 and then through this winding back to the otherterrninal of the source. The diagram shows at the upper left-hand portion the two brushes 3 and 3a of one brush assembly in engagement with two adjoining contact surfaces in one of the contact paths of the winding with an intermediate turn shown of less height, between the two turns engaged by the brushes. From the left-hand brush 3 the cable or lead wire ll extends around the lower leg la of the core in one turn 26 and is then connected to one terminal of its resistor 23, then through the resistor to form another turn 26a. around the lower leg of the core and then back to the other brush 3a in the manner already de scribed with reference to Fig. l of the drawings. From the connection between the resistor 23 and the turn 26 a lead 23a extends to the load circuit indicated as lamp bulbs 2! and thence to the upper terminal of the source .25. The turn 26 which is connected to the brush 3 on the higher voltage side of the main winding 2, is connected so that the electromotive force induced therein is subtractive with reference to the voltage applied to the main winding. The turn 26a which is connected to the brush 3a on the lower voltage side, is connected so that the electromotive force induced therein is additive with reference to the applied voltage. The electromotive forces induced in the turns .26 and 260. are additive to each other in the local circuit between the brushes 3 and 3a. A second pair of brushes of another brush assembly is shown in Fig. 8 in simultaneous engagement with a C011? tact surface of one of the turns of the main winding and the connections therefrom to another resistor 23 and to another load circuit 210. are similar to those already described with reference to the first considered brush assembly; and it will be understood that each of the other four brush assemblies would be similarly connected and are not shown in Fig. 8 in order to avoid confusion of the connections.

The change of voltage applied to the load circuit by any one of the brush assemblies. when it. is moved along its contact path on the winding, may now be described. First assume that the brush assembly is in any'selected position on its contact path with the two brushes thereof both in engagement with the contact surface of one turn of the main winding such as indicated in Fig. 6 and by the right-hand pair of brushes in Fig. 8. In that case the voltage applied to the load circuit will be approximately the voltage of a portion of the main winding. The two turns on the leg la of the core are then in a local circuit closed on themselves through the brushes and through the corresponding resistor 23 and the electromotive forces of the two turns are additive in this local circuit. This results in a local current through the .resistor. However, the heat generated in the resistor is dissipated there from and as shown in Fig. l the resistors are located at some distance from the core and from the main winding and are in a position to freely dissipate the heat. When the brush assembly aesassv is moved to the left in a direction to increase the voltage applied to the load circuit, the brush 3a remains in engagement with its contact surface of one turn of the main winding, whereas the brush 3 then engages insulation between turns. Under this condition the voltage applied to the load circuit is increased because the local brush assembly circuit is opened, the subtractive voltage of the turn 26 in the load circuit is removed and the additive voltage of the turn 26a is effective in increasing somewhat the voltage applied to the load circuit. When the brush assembly is moved in a direction to further increase the voltage applied to the load circuit, the next position of the brushes 3 and 3a will be that shown in Fig. 7 with the two brushes respectively engaging the contact surfaces of two turns of the main winding. This further increases the voltage applied to the load circuit because two turns of the main winding are now added in the load circuit from brush 3. In the next movement of the brush assembly the brush 3 will continue to engage its contact surface of the winding, whereas the brush 3a will then engage the insulation between contact surfaces. In this movement no appreciable change is obtained over that last considered because the only change is to open the circuit of the brush 3a. Further movement of the brush assembly in the direction considered will next result in both brushes 3 and 3a engaging the same contact surface of a turn of the main winding corresponding to the condition first described when the two brushes both engaged one turn of the main winding of lower voltage. Further movement repeats this sequence of operations with a succession of increased voltages obtained in passing from the condition when both brushes engage the same contact surface of one turn of the main winding to the condition when both brushes engage the contact surface of the next turn of the main winding. Thus an increased number of steps in change of voltage applied to the load circuit is obtained in passing from the contact surface of one turn of the main winding to that of the next turn. It is evident that when the brush assembly is moved in a direction to decrease the voltage applied to the load circuit, the reverse action from that already described results.

It should be noted that the connection from the auxiliary winding comprising the turns 26 and 26a, to the load circuit is from an intermediate portion thereof and at a point to include the resistor 23 on the low voltage side of the local brush circuit, that is, in the connections as shown, the resistor 23is on the same side of the tap connection to the load circuit as the turn 26a for obtaining the mode of operation described.

Fig. 9 is similar to Fig. 8 with the corresponding parts designated by the same reference characters as in Fig. 8, but Fig. 9 includes additional features of this improvement. In Fig. 8 it was assumed that the voltage of the supply lines from the source 25 was the usual 110 volt system; but where the available voltage from the source is higher, say 220 volts for supplying a 110 volt load circuit or circuits, the structure of the unit may have one of the added features of Fig. 9. In this case the winding 2 on the upper leg l of the core is assumed to be the same as that already described, but from the right-hand end of this winding the supply circuit is continued through another winding 28 on the lower leg Id of the. coreand extending over the full length thereof and having approximately the same number of turns as the winding 2. This additional winding is indicated in Fig. 9 as closely wound on the leg la and from the left-hand terminal a connection extends back to the source 25. The turns 26 and 26a in Fig. 9 correspond to the same turns of Fig. 8 connected to the resistors 23 in the same manner and are indicated in Fig. 9 as extending considerably beyond the turns 28 for clearness. From the junction of the turn 26 with one terminal of the resistor 23 an additional winding 29 of a few turns is included in the connection to the load circuit and the connections of these windings 29 is such as to increase the voltage applied to the load circuits for the purpose later explained. The turns of the windings 29 are indicated extending beyond the core more than the winding 28 and less than the windings 26 and 26a for clearness in tracing the circuits. In practice the windings 29 and 2t and 26a would preferably be applied over the winding 28. The terminals of the windings 29 are respectively connected to one side of their respective load circuits, the other sides of the load circuits being connected at the point 30 to the connection between the main winding 2 and the winding 28.

In Fig. 9 the same sequence of changes occur when adjusting the brush assemblies to different positions on the contact surfaces of the main winding 2 as that already described with reference to Fig. 8. The additional winding 28 on the same core as the other windings gives a unitary structure adapted for direct connection to a higher voltage source, such as a 220 volt supply.

In Fig. 9, as already explained, extra turns 29 are shown included in each of the individual load circuits and these windings are connected in. a manner to supply an increased voltage to the load circuits respectively. These extra windings in the individual load circuits are for the purpose of taking care of the voltage drop between the control unit and the loads when the brush assemblies are in positions to apply the fullvoltage to the load circuits, or any approach to the full voltage. When one or more of the brush assemblies is adjusted to their no voltage limit, the extra turns 29 in their respective load circuits would continue to supply a few volts thereto and in the case of a lamp load would keep the lamp filaments warm and result in a quicker response to an increase of voltage than otherwise and give smoother operation. Also by providing these extra turns in the individual load circuits, the main winding 2 is relieved from the necessity of such extra turns and permits the full length of the Winding 2 to be utilized for change of voltage. Another advantage of these extra turns being placed in the individual load circuits is that they need only to be of a capacity adapted for their individual loads. These extra turns 29 could, of course, be utilized in the load circuits of Fig. 8 if desired.

In Fig. 10 there is indicated in a general way another method of forming the main winding 2 on the core. Fig. 10 shows a cross section through the two legs l and id of the core and the main winding designated as 3! passes over leg I of the core, then downwardly between the two legs of the core under the leg la, then around leg la across the top thereof, then down between the two legs of the core under the leg I and then up around the outside of the leg I over the top thereof and then down between the two legs and so on. to the other terminal of the main winding 3|. This might be termed a figure eight form of winding. The top surfaces of the turns of the winding on each leg of the core have exposed contact surfaces on the adjacent turns and thus form two paths of the contact surfaces of the full width of each leg, giving considerable width to these paths which permits the use of a large number of brush assemblies on each path for supplying individual load circuits in the manner already described. Fig. indicates by the reference character 32 five brush assemblies on each path of contact surfaces. These brush assemblies would, of course, have connections to additional auxiliary windings and to impedance devices, such as resistors 23, as already described with. reference to Fig. 8 or Fig. 9. This structure also has the advantage that theheating at the contact surfaces in the two paths is widely displaced on the two legs of the core However, it has the disadvantage of difiiculty in applying the windings to the core.

In some cases where the width of the contact surfaces in the different structures already described is not sufiicient to accommodate all the desired brush assemblies, available width of the contact paths may be obtained by extending the contact paths outwardly to one side or to both sides of the paths. This may be accomplished by placing contact bars on top of the exposed surfaces of the windings and soldering, brazing or otherwise electrically connecting such bars to the contact surfaces of the turns of the windings. This form of structure is indicated in Fig. 11 which shows a winding 33 on a core section 34 and a contact bar 35 fixed and electrically connected to each exposed contact surface of the turns of the winding. This bar is shown extended on both sides of the winding thereby permitting an increased number of brush assemblies 32 being utilized, seven of such brush assemblies being indicated in Fig. 11.

In the foregoing disclosures such as particularly shown in Figs. 1, 8 and 9, there are two turns of the main winding between successive contact surfaces in each contact path; and there are two auxiliary turns 26 and 26a in series with each other and with an impedance device 23 inserted between them. In some cases there may be more than two turns in the main winding between successive contact surfaces thereof and in that case the total number of auxiliary turns 25 and 26a should preferably be equal thereto with onehalf in each portion 26 and 26a.

Instead of inserting the impedance device 23 in series between the turns of the auxiliary winding such as between the turns 26 and 25a, the winding 26 may in some cases be omitted and the total auxiliary turns included in the winding 25a. In that case the auxiliary windin would be the winding of 25a only with its turns equal in number to the number of turns between adjacent segments of the contact path; and connected in circuit in series with the resistor 23 to the brush of lower potential with its electrcmotive force in a direction to be additive to that of the applied voltage of the source, as already described with reference to Figs. 8 and 9. A somewhat similar change of increase in voltage applied to the load circuit in moving the brush assembly to a higher voltage position would then occur as already described with reference to Figs. 8 and 9, when the brush assembly is moved from a position where both brushes engage one contact segment to the position where both brushes engage the 10 next contact segment. However, the provision of the two windings 26and 26a as shown is preferable and is more convenient in the location and connections of the parts.

Also, instead of having the auxiliary turns, such as 25 and 260., on the main core of the transformer as shown, they could be applied on a small separate transformer with a number of turns thereon for each load circuit as would cause their electromotive force to equal approximately the electromotive force of the turns of the main winding between the two adjacent contact surfaces.

Although the invention is shown applied to an auto-transformer, it could obviously be applied to the secondary winding of the usual type of transformer in the same manner.

Instead of applying the brush assembliesdirectly to exposed contact surfaces of the turns of the main winding, taps from the main winding could be provided with contact surfaces to form one or more contact paths which would be engaged by the brush assemblies.

It will be understood that various mod rfi Q of this invention may be made according to particular requirements and at the choice of the designer without departing from the scope thereof.

I claim:

1. A variable voltage transformer comprising a core, a main winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and a resistor connected in series with each other in a local circuit between said brushes, and a tap connection from said local circuit to a load circuit of the transformer, said auxiliary winding and resistor being on the low voltage side of said tap connection.

2. A variable voltage transformer comprising a core, a, main winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and an impedance device connected in series with each other in a, local circuit between said brushes, and a tap connection from between portions of said auxiliary winding to a load circuit of the transformer, the said impedance device being on the low voltage side of said tap connection.

3. A variable voltage transformer comprising a core, a main winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and an impedance device connected in series with each other in a local circuit between said brushes, and a tap connection from between portions of said auxiliary winding to a load circuit of the transformer, the said impedance device being on the low voltage side of said tap connection, the said portion of the winding on said low voltage side being connected to have its electromotive force additive and the other portion subtractive with reference to the electromotive force applied to the main winding.

4. A variable voltage transformer comprising a core, a main winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and an impedance device connected in series with each other in a local circuit between said brushes, and a tap connection from between portions of said auxiliary winding to a load circuit of the transformer, the number of turns of said auxiliary winding between said brushes being such that the electromotive force induced therein is approximately equal to that induced in the turns of said main winding connected between adjacent contact surfaces.

5. A variable voltage transformer comprising a core, a main winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and an impedance device connected in series with each other in a local circuit between said brushes, and a tap connection from between portions of said auxiliary winding to a load circuit of the transformer, the number of turns of said auxiliary winding between said brushes being such that the electromotive force induced therein is approximately equal to that induced in the turns of said main winding connected between adjacent contact surfaces, the said impedance device being on the low voltage side of said tap connection, the said portion of the winding on said low voltage side bein connected to have its electromotive force additive and the other portion subtractive with reference to the electromotive force applied to the main winding.

6. A variable voltage transformer comprising a core, a main winding thereon having two paths of contact surfaces side by side, the contact surfaces of one of the paths being on different turns of the winding from that of the other path, a brush assembly movable as a unit along one of said paths, a load circuit connected to said brush assembly and to the transformer winding, a second brush assembly movable along the other of said paths, and a second load circuit connected to said second brush assembly and to the transformer independently of said first brush assembly and of its load circuit for obtaining independent control of the load circuits by independent movement of each of said brush assemblies, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said paths, an auxiliary winding and an impedance device connected in series in a local circuit between the brushes of each of said brush assemblies respectively, said load circuits respectively having a tap connection from between portions of said auxiliary windings respectively.

'7. A variable voltage transformer unit for controlling a plurality of load circuits independently of each other comprising a frame support, a core at the upper portion of said support, a main winding on an upper leg of said core having exposed contact surfaces forming a contact path, a plurality of brush assemblies each movable as a unit along said path connected to respective load circuits for varying the voltage applied thereto independently of each other, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said path, a plurality of resistors mounted on a lower portion of said support, and a plurality of auxiliary windings on a lower leg of said core, each of said auxiliary windings being connected respectively in series with one of said resistors in a local circuit between the two I rushes of each brush assembly respectively.

8. A variable voltage transformer unit for controlling a plurality of load circuits independently of each other comprising a frame support, a core at the upper portion of said support, a main winding on an upper leg of said core having exposed contact surfaces forming a contact path, a plurality of brush assemblies each movable as a unit along said path connected to respective load circuits for varying the voltage applied thereto independently of each other, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said path, a plurality of resistors mounted on a lower portion of said support, and a plurality of auxiliary windings on a lower leg of said core, each of said auxiliary windings being connected respectively in series with one of said resistors in a local circuit between the two brushes of each brush assembly respectively, the connection to the respective load circuits from said brush assemblies respectively being from an intermediate portion of said auxiliary windings respectively.

9. A variable voltage transformer comprising a core, a winding thereon having two paths of contact surfaces side by side, the contact surfaces of one of the paths being on different turns of the winding from that of the other path, said paths being formed on one side of the core by passing an insulated portion of a turn of the winding under a portion of another turn of the winding which has an exposed contact surface, a brush assembly movable along one of said paths, a load circuit connected to said brush assembly and to the transformer winding, a second brush assembly movable along the other of said paths, and a second load circuit connected to said second brush assembly and to the transformer independently of said first brush assembly and of its load circuit for obtaining independent control of the load circuits by independent movement of each of said brush assemblies, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said paths, an auxiliary winding for each of said brush assemblies having its terminals connected respectively to the brushes of its brush assembly, an impedance device connected in series with each of said auxiliary windings respectively between its respective brushes, the connection to the load circuits respectively being from an intermediate portion of each of said auxiliary windings.

10. A variable voltage transformer comprising a core, a winding thereon having two paths of contact surfaces side by side, the contact sur faces of one of the paths being on different turns of the winding from that of the other path, said winding being formed on two legs of the core by the turns passing from the top of each leg down between the legs under the opposite leg and then over the top of the said opposite leg and the two paths of contact surfaces being formed on one side of each of the legs, a brush assembly movable along one of said paths, a load circuit connected to said. brush assembly and to the transformer winding, a second brush assembly movable along the other of said paths, and a second load. circuit connected to said second brush assembly and to the transformer independently of said first brush assembly and of its load circuit for obtaining independent control of the load circuits by independent movement of each of said brush assemblies.

11. A variable voltage transformer comprising a core, a winding thereon having two paths of contact surfaces side by side, the contact surfaces of one of the paths being on different turns of the winding from that of the other path, said winding being formed on two legs of the core by the turns passing from the top of each leg down between the legs under the opposite leg and then over the top of the said opposite leg and the two paths of contact surfaces being formed on one side of each of the legs, a brush assembly movable along one of said paths, a load circuit connected to said brush assembly and to the transformer winding, a second brush assembly movable along the other of said paths, and a second load circuit connected to said second brush assembly and to the transformer independently of said first brush assembly and of its load circuit for obtaining independent control of the load circuits by independent movement of each. of said brush assemblies, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said paths, an auxiliary winding for each of said brush assemblies having its terminals connected respectively to the brushes of its brush assembly, an impedance device connected in series with each of said auxiliary windings respectively between its respective brushes, the connection to the load circuits respectively being from an intermediate portion of each of said auxiliary windings.

12. A variable voltage transformer having a main winding with portions of its turns exposed to form a path of spaced contact surfaces and having extensions beyond the width of said path to form wide spaced contact surfaces for engagement by a plurality of brush assemblies independently movable along the length of said path, each having two brushes insulated from each other and spaced from each other in fixed relation in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding and an impedance device connected in series with each other in a local circuit between said brushes, and a top connection from between portions of said auxiliary winding to a load circuit of the transformer, the said impedance device being on the low voltage side of said top connection, the said portion of the winding on said low voltage side being connected to have its electromotive force additive and the other por- 14 tion subtractive with reference to the electromotive forced applied to the main winding.

13. A variable voltage transformer having a main winding in two sections in series with each other for connection to a source, each of said sections having approximately-the same number of turns as the other section, one of said sections being wound on one leg of the transformer core and the other of said sections being wound on another leg of said core, one of said sections having turns provided with spaced contact surfaces to form a contact path, a plurality of brush assemblies movable along said path, each of said brush assemblies being movable independently of the others, each of said brush assemblies having two brushes insulated from each other and spaced from each other in the direction of said paths, an auxiliary winding on said core for each of said brush assemblies having its terminals connected respectively to the brushes of its brush assembly, an impedance device connected in series with each of said auxiliary windings respectively between its respective brushes, load circuits connected respectively from an intermediate portion of each of said auxiliary windings to a terminal between said sections, each of said individual load circuits having an auxiliary winding on said core respectively for increasing the voltage applied to its respective load circuit, and each of said load circuits being connected to said first named auxiliary windings respectively with said impedance device on the low voltage side.

14. A variable voltage transformer comprising a core, a winding thereon having turns provided with spaced cont-act surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding having its terminals connected respectively to said brushes, and a connection from said auxiliary winding to a load circuit of said transformer, said brush assembly having a main portion of insulating material, each of the brushes being of sheet metal and having a contact portion at its lower edge and extending upwardly within said insulating portion and having a bent portion at its upper end, a spring engaging said bent portion to impose pressure on the brushes against the contact path and said terminal contacting each of said respective brushes between said bent portion and said lower edge.

15. A variable voltage transformer comprising a core, a winding thereon having turns provided with spaced contact surfaces to form a contact path, a brush assembly movable along said path and having two brushes insulated from each other and spaced from each other in the direction of said path, at least one of said brushes always being in engagement with one of said contact surfaces as said brush assembly is moved along said path, an auxiliary winding having its terminals connected respectively to said brushes by flexible cables, spaced partitions for guiding said cables, and a connection from an intermediate portion of said auxiliary winding to a load circuit of the transformer, said brush assembly having a main portion of insulating material, each of the brushes being of sheet metal and having a contact portion at its lower edge and extending upwardly within said insulating 15 16 portion and. having a bent portion at its upper UNITED STATES PATENTS end, a spring engaging said bent portion to im- Number Name Date pose pressure on the brushes against the contact 2 186 211 schwaiger Jan 9 1940 path and said terminal contacting each of said Logan 1940 respective brushes between said bent portion and 5 2361384 Davis Oct. 1944 Sam 1Wer edge- 2,488,370 Boelens et a1. Nov. 15, 1949 ARIEL R. DAVIS.

REFERENCES CITED The following references are of record in the 10 file of this patent: 

